Bath oil and similar skin conditioners are water soluble or water dispersible materials of fairly high viscosity which have been traditionally added in small quantities to bath water. It has been proposed in the past to mix bath oil with water passing through a shower head by aspirating it into the shower line at a point upstream of the head. Such systems have not found wide acceptance because they are extremely sensitive to water flow rate, since they rely on the differential between atmospheric pressure and the pressure of the flowing water at the point of aspiration, and this differential is quite small at low flow rates. In addition, the line constriction required to produce good aspiration objectionably limits the maximum delivery rate of the shower line. Furthermore, the small lines involved in aspiration systems are prone to clogging.
An improvement over aspirated systems is the positive displacement device shown in Headen et al. U.S. Pat. No. 4,121,773, and in part in the prior art cited therein, namely: McPherson U.S. Pat. No. 2,058,901; Brunner U.S. Pat. No. 2,235,278; McNair et al U.S. Pat. No. 3,132,806; Hubert U.S. Pat. No. 3,194,444; James U.S. Pat. No. 3,764,074 and Lemond U.S. Pat. No. 3,847,354. But even with such systems, extreme care is required in metering, inasmuch as very small changes in area through the metering valve cause significant changes in the length of time consumed in dispensing a single containerfull of bath oil. (Generally, the dispensing should occupy about a minute, to allow for convenient even distribution over the body. Dispensing times of only a few seconds do not allow this, while long dispensing times cause impatience and waste hot water.)